Re: [Valgrind-developers] allowing skins to instrument conditional jumps

[Nicholas N. <nj...@ca...>]

On 30 Sep 2002, Jeremy Fitzhardinge wrote:

> I'm writing a skin to generate gprof-like output, so I need to see all
> the edges in the control flow graph. In particular, I'd like to insert
> some instrumentation code which is run IFF a conditional branch is
> taken.
>
> I see a few options:
>         * something to properly represent uInstr sequences with
>           conditionals within the ucode for one real instruction (ie,
>           some way of representing jumps to real addresses rather than
>           simulated addresses).  Sounds messy.
>         * Intercept the jump target address and generate a completely
>           new piece of code at some place within the simulated address
>           space.  Ugly.
>         * Introduce a new exceptional value for ebp when it is passed
>           back into the dispatcher to trigger a call into the skin.
>           Would need some way to attach some kind of argument values for
>           the call (encode in %edx?).  Seems like the least nasty.
>
> Any opinions?

Best way I can think of doing it, which only requires skin changes rather
than core changes, is this:  using the `extended_UCode' need, add a new
UInstr PRE_JCC, which gets inserted by SK_(instrument) before conditional
JMPs, evaluates the condition, and calls a C function (or whatever) if
it's true.  This would duplicate the condition evaluation but that
shouldn't matter since they're trivial (just checking an EFLAGS bit I
think).

It's a bit nasty that something as simple as this requires a new UInstr...

Oh, and apologies for the delay in replying.

N